The use of the cardiac catheter as a tool for laboratory and clinical investigation has opened a new era in the understanding of many physiologic and clinical aspects of cardiovascular function and disease. This has been particulary true in the field of congenital heart disease. There have been a few attempts to perform therapeutic procedures during cardiac catheterization. Our investigations of technics to produce a palliative atrial septostomy using a balloon-tipped catheter has been one of the most productive areas of this type of study. We propose to continue to improve this procedure, and to explore new therapeutic uses for the cardiac catheter. Among these proposed adaptations are - 1) using the catheter as a means of introducing detachable prostheses for closure of septal defects and patent ductus arteriosus, 2) developing self-seating prostheses for rapid defect closure during cardiac surgery, 3) improved transcatheter atrioseptostomy technics.